Omeprazole! - My wife had migraine auras without headaches for a number of years. I know it does not sound logical but we tied it to her Proton Pump Inhibitor use - Omeprazole. She started taking a larger dose of Omeprazole and shortly afterwards started having several aura episodes each week. After many years of trying to find a cure we ran across an article from Harvard Medical linking Omeprazole to brain issues including dementia. Once she stopped taking the Omeprazole her Migraine auras went down to one every several months. A neurologist she was seeing was extremely skeptical at first but then about a year later he praised her for this discovery and confirmed the cause/effect relationship between auras and Omeprazole (for her at least).
If you are on a PPI you may want to try stopping for a while to see if that helps.